I honestly don’t think it matters whether he thinks your reaction is unreasonable or not. Everyone deserves to feel secure in their relationship, and you’re telling him this friendship is making you feel uncomfortable and unsettled. That should matter.
For me, the bigger issue isn’t even the friendship itself (if it is as innocent as he says), it’s that he’s not really acknowledging how it’s affecting you. That’s the part that would be most upsetting and was for me in that very same situation.
If it genuinely is just a friendship, then there should absolutely be a way for him to have that and for you to still feel secure and prioritised at the same time. In my scenario I was made to feel shit about myself for even questioning it. I was the one with a problem not them.
I’d trust your instincts in terms of how it’s making you feel, not necessarily that something is definitely going on, but that something about the situation isn’t sitting right for you, and that’s worth paying attention to.
Maybe a final conversation to firmly say that the way he has handled this whole situation, not the friendship itself, has made you question the relationship and you are seriously contemplating the future of your marriage and is only fair he understand how serious this has become for you. His reaction to that conversation will tell you everything you need to know.
In my situation my instincts were correct, I think for him he genuinely thought they were ‘besties’ she was actually convinced he was the love of her life 🙄